The present invention relates in general to power supply circuitry, and in particular to method and circuitry for implementing programmable active droop for power supply voltage regulators.
A particular type of voltage regulator is a switching voltage regulator. A simplified schematic of a switching voltage regulator 10 is shown in FIG. 1. A periodic signal, 100 is applied to a high-side driver 102 and a low-side driver 104, which in turn drive high-side switch 106 and low-side switch 108, respectively. Inverter 110 ensures that low-side switch 108 remains off when high-side switch 106 is on. When high-side switch 106 is on (low-side switch 108 off), current flows through inductor 112 and stores energy in a magnetic field of inductor 112. When low-side switch 108 turns on, the energy stored in inductor 112 is transferred to a filter capacitor 114 at an output 116 of the switching regulator 10. Filter capacitor also functions to smooth the signal at output 116. The output signal is, therefore, nearly a DC voltage and, for a periodic square wave input signal, this output signal is directly proportional to the duty cycle of the input signal.
Modern power supplies are often required to have a very tightly regulated output voltage even during transients, which are caused, for example, by sudden changes in load current demands. When a switching regulator is supplying a large current and then is suddenly required to supply a substantially smaller current, voltage transients at the output may be large enough to exceed the acceptable voltage output limit. To counter this problem switched voltage regulators sometimes include a droop function, whereby the output voltage of the power supply is lowered when the output load current increases, so that there is additional headroom for transients. In such a system, when the output load transitions from heavy to light (i.e., high output current to low output current), the voltage is initially towards the low end of its acceptable range. The current output from the power supply""s inductor (which takes time to decrease both because of its reactance and the delay in the power supply""s control loop) charges the output capacitors of the power supply, and the output voltage may increase further without hitting the high end of the acceptable range compared with the output voltage if the initial voltage was not lowered. The same effect is true in reverse, i.e. for light to heavy load conditions. The net effect is that the number of output capacitors the power supply requires to meet the transient load regulation is decreased, thereby decreasing cost and circuit size.
The improvement in transient response is commonly realized by first adding a constant positive offset voltage to the output of the power supply when operating at no load current, so as to maximize the voltage headroom when the load current increases. The droop is then typically accomplished passively, by placing a series resistor at the output of power supply so that as the load current increases, the output voltage drops. The problem with this xe2x80x9cpassivexe2x80x9d approach is that the efficiency of the regulator is compromised due to power dissipated by the series resistor.
Droop can also be generated actively, for example by measuring the current and amplifying the resultant signal, which may then be used to adjust the output voltage the power supply is producing. The current measurement can be accomplished in a variety of ways, for example with a series resistor, or by measuring the voltage drop across one of the MOSFET power switches, or by averaging the voltage drop across the inductor, etc. Although the latter two schemes do not need to use additional components to implement droop, they have the problem that the amount of droop is set by Ixc3x97R, where I is the current and R is the resistance of the element being used for current sensing. Once the element is fixed, a change in the load design of the power supply, or in the amount of droop required, cannot be accommodated, since the droop is fixed by a constant Ixc3x97R. Additionally, any temperature coefficient in the sense resistor R will be reflected as a temperature coefficient in the droop.
What is needed, therefore, are improved methods and circuitry for implementing droop in power supply output voltages.
The present invention provides method and circuitry for implementing active droop eliminating lossy sensing, while at the same time permitting the amount of droop, as well as the droop temperature coefficient, to be programmed arbitrarily, by using an additional control element.
In a first aspect of the invention, a switching regulator having a droop function is disclosed. An exemplary switching regulator comprises: a high-side switch having a gate coupled to a regulator input configured to receive an input signal, a drain coupled to a power supply and a source; a low-side switch having a gate configured to receive an inverse of the input signal, a drain coupled to the source of the high-side switch and a source coupled to ground; an inductor having a first end coupled to the drain of the low-side switch and a second end providing an output voltage; and an amplifier having inputs coupled to the drain and source of the high-side switch (or the source and drain of the low-side switch) and an output operable to provide a droop voltage signal.
In a second aspect of the invention, a method of providing a droop signal is disclosed. An exemplary method comprises the steps of: A method of maintaining an output voltage of a switching regulator within a voltage range, the method comprising the steps of: measuring a current through a high-side switch (or low-side switch) of the switching regulator; and providing droop signal that is operable to lower an output voltage of the switching regulator as a linear function of the current measured through the switch exceeds a predetermined threshold.
The following detailed description and the accompanying drawings provide a better understanding of the nature and advantages of the present invention.